Micromechanical structures are generally familiar. For example, German Patent Application No. 10 2006 047 135 A1 describes a yaw-rate sensor having two identical structures, each structure having two driving-mass elements which are joined to a substrate via four driving-mass springs, and which in each case are joined to a sensing-mass element via eight sensing-mass springs. The two sensing-mass elements are coupled to each other by a coupling spring. The two driving-mass elements are driven to oscillate along a first axis parallel to the surface of the substrate, the sensing-mass elements being displaceable along a second axis perpendicular to the surface under the effect of Coriolis forces. The yaw-rate sensor also has sensing devices below the sensing-mass elements, by which the displacements of the sensing-mass elements are detectable, so that the yaw-rate sensor is provided to detect a yaw rate about an axis of rotation perpendicular to the first and to the second direction.
Generally, the mass structures are provided with perforation holes to permit the exposure of the movable structures (first and second sensing elements, first and second driving elements), particularly in an etching process such as vapor-phase etching. Thus, in the case of a given layer thickness, the mass of the mass structures is a function of the line width of the mass lines and the size of the perforation holes. Production-induced variations in the trench widths of the spring lines and mass lines, hereinafter known as edge-loss spreads, disadvantageously lead to frequency spreads of the respective spring-mass system. The conventional yaw-rate sensor has the further disadvantage that, because the sensing device in the form of electrodes is disposed on one side below the sensing-mass elements, there is the risk that the sensing-mass elements will be pulled downward in the direction of the electrodes in response to the application of a positive-feedback voltage and a quadrature-compensation voltage. At very high voltage levels, this can even lead to electromechanical instability, what is termed “snapping”, in which the sensing-mass elements are pulled completely onto the electrodes. In practice, the positive-feedback voltage compensates for a frequency allowance between the driving mode and detection mode of the yaw-rate sensor, so that the yaw-rate sensor may be operated fully resonantly. The frequency allowance set is used to compensate for manufacturing tolerances; hereinafter, the frequency allowance is also denoted as frequency splitting. Moreover, mechanical and electrical crosstalk of the drive movement in the detection path is suppressed by the quadrature-compensation voltage. In this context, the susceptibility of the yaw-rate sensor to vibrate is proportional to the quadrature and to the frequency splitting, so that as low a quadrature as possible and the lowest possible frequency splitting are necessary to achieve as low a vibrational susceptibility as possible. However, in the case of the yaw-rate sensor, the suspension springs for securing the sensing-mass elements are mainly subject to bending stress in response to a displacement, the bending stiffness of the suspension springs increasing in proportion to the third power of the functional-layer thickness, while the mass increases only linearly with the functional-layer thickness. The result is that the detection frequency exhibits a great dependency on the thickness of the functional layer, while the drive frequency does not vary or varies only negligibly with the functional-layer thickness. For fully resonant operation of the yaw-rate sensor, it is therefore disadvantageous that a comparatively great frequency splitting must be set in order to compensate for manufacturing tolerances in the production of the yaw-rate sensor. However, the vibrational susceptibility described above is thereby increased, as well.